For example, content data such as a movie and music is recorded onto a DVD. Such content data is digital data. Thus, even if the content data is copied, the data itself does not deteriorate. As a result, if one content data to be a source is obtained, it is possible to make many copies of the content data with the same content, and furthermore, using the copied content data as a source, it is possible to make more copies of the content data with the same content.
Thus, with respect to the information recording medium, such as a DVD, on which the digital data is recorded, measures for restricting the copying are taken for copyright protection. The most general measure of them is to encrypt the content data.